Abstract
Pumpkin leaves grown under high light (500-700 micromol of photons m-2.s-1) were illuminated under photon flux densities ranging from 6.5 to 1500 micromol.m-2.s-1 in the presence of lincomycin, an inhibitor of chloroplast protein synthesis. The illumination at all light intensities caused photoinhibition, measured as a decrease in the ratio of variable to maximum fluorescence. Loss of photosystem II (PSII) electron transfer activity correlated with the decrease in the fluorescence ratio. The rate constant of photoinhibition, determined from first-order fits, was directly proportional to photon flux density at all light intensities studied. The fluorescence ratio did not decrease if the leaves were illuminated in low light in the absence of lincomycin or incubated in darkness in the presence of lincomycin. The constancy of the quantum yield of photoinhibition under different photon flux densities strongly suggests that photoinhibition in vivo occurs by one dominant mechanism under all light intensities. This mechanism probably is not the acceptor side mechanism characterized in the anaerobic case in vitro. Furthermore, there was an excellent correlation between the loss of PSII activity and the loss of the D1 protein from thylakoid membranes under low light. At low light, photoinhibition occurs so slowly that inactive PSII centers with the D1 protein waiting to be degraded do not accumulate. The kinetic agreement between D1 protein degradation and the inactivation of PSII indicates that the turnover of the D1 protein depends on photoinhibition under both low and high light.
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Claims 11
Claim · model-written summary
Illumination caused photoinhibition at every studied light intensity.
- Taxon
- Pumpkine0
- Treatment
- illumination at all light intensitiese0
- Outcome
- ratio of variable to maximum fluorescencee0
- Direction
- decreasee0
- Qualifier
- presence of lincomycine0
Not reportedComparator
Verbatim
e0“Pumpkin leaves grown under high light (500-700 micromol of photons m-2.s-1) were illuminated under photon flux densities ranging from 6.5 to 1500 micromol.m-2.s-1 in the presence of lincomycin, an inhibitor of chloroplast protein synthesis. The illumination at all light intensities caused photoinhibition, measured as a decrease in the ratio of variable to maximum fluorescence.”
Claim · model-written summary
Loss of PSII electron transfer activity correlated with declining fluorescence ratio.
Verbatim
e0“Loss of photosystem II (PSII) electron transfer activity correlated with the decrease in the fluorescence ratio.”
t0“leaves”
e1“Pumpkin leaves grown under high light”
Claim · model-written summary
The photoinhibition rate constant was directly proportional to photon flux density.
Not reportedDirection
Verbatim
e0“The rate constant of photoinhibition, determined from first-order fits, was directly proportional to photon flux density at all light intensities studied.”
e1“Pumpkin leaves grown under high light”
Claim · model-written summary
The fluorescence ratio did not decrease under low light without lincomycin.
Verbatim
e0“The fluorescence ratio did not decrease if the leaves were illuminated in low light in the absence of lincomycin or incubated in darkness in the presence of lincomycin.”
e1“Pumpkin leaves grown under high light”
Claim · model-written summary
The fluorescence ratio did not decrease in darkness with lincomycin.
Verbatim
e0“The fluorescence ratio did not decrease if the leaves were illuminated in low light in the absence of lincomycin or incubated in darkness in the presence of lincomycin.”
e1“Pumpkin leaves grown under high light”
Claim · model-written summary
Quantum yield of photoinhibition remained constant across photon flux densities.
Verbatim
e0“The constancy of the quantum yield of photoinhibition under different photon flux densities strongly suggests that photoinhibition in vivo occurs by one dominant mechanism under all light intensities.”
e1“Pumpkin leaves grown under high light”
Claim · model-written summary
The authors infer one dominant in vivo photoinhibition mechanism across light intensities.
Verbatim
e0“photoinhibition in vivo occurs by one dominant mechanism under all light intensities”
e1“Pumpkin leaves grown under high light”
Claim · model-written summary
The authors consider the in vivo mechanism probably distinct from the anaerobic acceptor-side mechanism.
Verbatim
e0“This mechanism probably is not the acceptor side mechanism characterized in the anaerobic case in vitro.”
e1“Pumpkin leaves grown under high light”
Claim · model-written summary
Loss of PSII activity correlated strongly with D1 protein loss under low light.
Verbatim
e0“there was an excellent correlation between the loss of PSII activity and the loss of the D1 protein from thylakoid membranes under low light.”
e1“Pumpkin leaves grown under high light”
Claim · model-written summary
Inactive PSII centers with D1 awaiting degradation did not accumulate under low light.
Verbatim
e0“At low light, photoinhibition occurs so slowly that inactive PSII centers with the D1 protein waiting to be degraded do not accumulate.”
e1“Pumpkin leaves grown under high light”
Claim · model-written summary
D1 protein turnover was inferred to depend on photoinhibition at low and high light.
Verbatim
e0“The kinetic agreement between D1 protein degradation and the inactivation of PSII indicates that the turnover of the D1 protein depends on photoinhibition under both low and high light.”
e1“Pumpkin leaves grown under high light”